It is known to provide a torque sensor apparatus in, for example, an electric power steering unit. The torque sensor apparatus senses a change in the strength of a magnetic field, which is caused by rotation of a rotatable shaft, with a magnetic sensor(s) to sense a shaft torque applied to the rotatable shaft. The magnetic sensor(s) is held between two magnetic flux collecting bodies. Particularly, in a case where the magnetic sensor(s) is held between two magnetic flux collecting portions of the magnetic flux collecting bodies, in which the magnetoresistance is relatively low, the magnetic flux is concentrated in the magnetic flux collecting portions. The magnetic flux, which is collected in one of the magnetic flux collecting portions, is conducted from the one of the magnetic flux collecting portions to the other one of the magnetic flux collecting portions, and this magnetic flux is sensed with the magnetic sensor(s).
For example, JP2007-263871A (US2007/0240521A1) teaches a structure that includes two magnetic flux collecting portions (magnetism collecting portions), which correspond to two magnetic sensors, respectively, and are formed in a magnetic flux collecting ring (magnetic flux collecting body). Alternatively, JP2008-232728A and JP2009-080020A (US2009/0078058A1) teach a structure that includes three magnetic flux collecting portions, which correspond to three magnetic sensors, respectively, and are formed in a magnetic flux collecting ring. Further alternatively, JP2006-38767A (US2006/0021451A1) teaches a structure that includes a rectangular magnetic flux collecting portion, which radially outwardly projects from a magnetic flux connecting ring, and two magnetic sensors are arranged adjacent to each other in the magnetic flux collecting portion.
The magnetic flux, which is collected in the magnetic flux collecting portion, is conducted not only to the magnetic sensor. Specifically, the magnetic flux, which is collected in the magnetic flux collecting portion, is also released into the surrounding space, which surrounds the magnetic flux collecting portion. For example, in the structure of JP2007-263871A (US2007/0240521A1), a portion of the magnetic flux, which is collected by the magnetic flux collecting ring, is released into the space located between the two magnetic flux collecting portions. The magnetic flux, which is released into the space, becomes a leaked magnetic flux that is not conducted to the magnetic sensor. Thereby, the amount of the magnetic flux, which can be effectively sensed with the magnetic sensor, is reduced by the amount of the leaked magnetic flux. Furthermore, in the structure of JP2008-232728A and JP2009-080020A (US2009/0078058A1), the number of the spaces defined among the magnetic flux collecting portions, is two. Therefore, the amount of the leaked magnetic flux is further increased.
In contrast, in the structure of JP2006-38767A (US2006/0021451A1), the two magnetic sensors are placed adjacent to each other in the single magnetic flux collecting portion. Therefore, the leakage of the magnetic flux into the space adjacent to the magnetic flux collecting portion can be reduced. However, the magnetic flux is also released into the surrounding space from a connecting section, which connects between the magnetic flux collecting ring and the sensor mounting section, so that the released magnetic flux becomes the leaked magnetic flux. In the structure of JP2006-38767A (US2006/0021451A1), the rectangular magnetic flux collecting portion has a relatively wide connecting section, which corresponds to the two magnetic sensors. Therefore, the amount of the leaked magnetic flux, which leaks from this connecting section to the surrounding space, is increased. Furthermore, in the case where the rectangular magnetic flux collecting portion is formed integrally into the magnetic flux collecting ring, the formation of the magnetic flux collecting ring into a circular form becomes difficult.